Dry offset printing method



P 19-65 H. J. WOLBERT 3,204,557

DRY OFFSET PRINTING METHOD Filed June 6, 1965 INVENTOR. f/A/QR/S f/l) W0155W BY p62:

United States Patent 3,204,557 DRY (DFFSET PRINTING METI-IDD Harris Jay Wolbert, Prospect Heights, Ill., assignor to American Can Company, New York, NFL, a corporation of New Jersey Filed June 6, 1963, Ser. No. 285,943 1 Claim. (Cl. 101-426) The present invention relates to printing on a metallic surface and more particularly to a dry offset method and apparatus which may be utilized to consistently produce sharply detailed prints of uniform quality on metal.

It has been the general practice in metal decorating, where the printing plates used are initially flat and are bent to conform to the surface of cylindrical printing cylinders, to utilize the wet offset, lithographic process for printing on a metallic surface. While this process has generally served the purpose, it has not proven entirely satisfactory for the reason that it requires a highly skilled pressman to control the water-ink balance on the printing plate in order to insure uniformity of printing quality. Even with such control, there are variations in printing quality owing to the dependency of this process on human judgment. Also, with the wet offset process, various types of improved inks cannot be used because of their water sensitivity, and it has not been possible to produce an ink film of sufficient thickness for certain purposes, especially in the case of white inks where a thin ink film produces an image having a grayed appearance. This deficiency is so obvious that frequently, in multicolor printing of can body labels, the white ink is applied in a coating operation rather by lithography.

The dry offset printing process requires no water to control the application of ink to the printing plate, since the image areas of the plate, being raised rather than being undercut as in the wet offset process, pick up the ink directly from the inking rollers. This process, therefore, is not subject to the disadvantages described above with respect to the wet offset process, and is capable of producing sharply detailed prints of uniform quality. Attempts have heretofore been made to utilize the dry offset process for printing on metallic surfaces, such as tinplate to be used in producing containers, but they have generally failed, primarily because printing plates with metallic printing surfaces have been used, and the raised metallic printing surfaces of such plates have been incapable of transferring sufficient ink to the inking blanket and thus to the tinplate. Also, because of the relative inflexibility of metallic printing plates, certain unetched portions of the printing surfaces, such as lettering, etc. are relatively stiffer than the etched portions, and become chordal relative to the printing cylinder surfaces on which the plates are mounted rather than becoming concentric herewith, thereby resulting in non-uniform pressure between these printing plate surfaces and the blanket to produce an uneven transfer of ink to the blanket and to the tinplate. These disadvantages thus have heretofore prevented the use of the dry offset process for printing on metallic surfaces such as tinplate. Because of the recognized practical advantages of the dry offset process over the wet offset process, a need has been created in the printing industry for an improved dry offset process for printing on metal.

The general purpose of the present invention, therefore, is to fulfill this need by providing a dry offset process for printing on metal which embraces all of the advantages of the conventional dry offset process and possesses none of the aforedescribed disadvantages. Briefly, this is accomplished by utilizing a plastic printing surface on a wrap-around printing plate. The plastic surface serves to transfer more ink to the blanket and to ice the metal, and, because of its flexibility, the plastic surface becomes concentric with the surface of the printing cylinder on which it is mounted. Since sufficient ink is transferred to the metal and because there is uniformity of pressure between the printing plate and the blanket, sharply detailed and uniform prints can be provided on metal by the instant dry offset process.

An object of the present invention is the provision of a dry offset printing apparatus and method which may be utilized to produce a quality print on a metallic surface, and which are suitable both for half-tone and line printing.

Another object is to provide such an apparatus which is capable of transferring a sufliciently heavy film of ink to the blanket and the metallic surface to thereby print a sharply detailed image of superior color thereon.

A further object of the invention is to provide a dry offset printing method which utilizes a printing plate which is concentric with the surface of the printing cylin der on which it is mounted so as to uniformly engage the blanket and which is capable of producing an image of uniform color strength on the metallic surface.

Numerous other objects and advantages of the invention will be apparent as it is better understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment thereof.

Referring to the drawing:

FIG. 1 is a schematic view of dry offset. printing apparatus constructed according to the principles of the present invention;

FIG. 2 is an enlarged perspective view of: a portion of the printing plate and printing cylinder illustrated in FIG. 1; and

FIG. 3 is an enlarged sectional view taken substantially along line 33 of FIG. 2.

As a preferred or exemplary embodiment of the invention, FIG. 1 illustrates a dry offset printing apparatus comprising a series of conventional inking form rollers 10 particularly adapted to receive ink from an ink fountain (not shown) and to distribute the ink evenly to a printing roll generally designated by the numeral 12. The printing roll 12 comprises a rotatable base cylinder 14- of conventional construction and a printing plate 16 which is firmly wrapped around the cylinder 14 and retained thereon under tension by a conventional gripping and tensioning element such as a clamp 18. The printing plate 16 has on its outer surface raised areas 17 representing the image to be reproduced, and a film of ink 19 (see FIG. 3) is transferred from these raised areas 17 to a blanket 20 which is mounted on a rotatable blanket cylinder 22. and retained thereon by a gripping and tensioning element 24,. The blanket 20 preferably is made of three ply rubber, while the blanket cylinder is of conventional construction and is made of cast iron or the like. Suitable apparatus, which generally comprises a drive motor and gear trains (not shown), is provided for driving the printing and blanket cylinders in opposite directions in a oneto-one relationship and suitable bearings (not shown) are provided for adjustably positioning them in a predetermined relationship to each other, in the conventional manner.

The image transferred from the raised areas 17 on the printing plate 16 to the blanket 2-0 is reproduced on a metallic sheet 26, such as tinplate to be used in making containers, by passing the sheet 26 between the blanket cylinder 22 and a conventional impression or back-up cylinder 28. A plurality of sheets 26 may be conventionally fed in timed relationship from a stack thereof (not shown) by a feeding mechanism which includes a reciprocating feed bar 3f having fingers 32 for advancing the sheets along a feed table 34. The sheets 26 are then 'advanced through the nip of the oppositely rotating blanket and impression cylinders 22, 28 respecti ely at which point the sheets 26 receive the ink impression from the blanket 20. As the sheets 26 pass through this nip they are fed onto a discharge conveyor 36 by which they may be carried through a drying zone and/ or for stacking or further disposition. If desired, the sheets 26 may be consecutively passed through additional printing apparatus similar or identical to that described herein to receive further ink impressions, as in multicolor printing where several ink impressions are deposited on the same sheet, either in overlapping or non-overlapping relationship.

Referring now to FIGS. 2 and 3, the printing plate 16 comprises an inner or base layer 38 of metal such as stainless steel, and an outer layer 4 9 of a plastic material which is secured to the base layer 38 by an adhesive layer 42. A suitable printing plate of this type is manufactured by E. I. du Pont de Ncmours & C0. under the tradename of Dycril, and by Chemical Products Corp. under the tradename of Chem-O-Type. The plastic layer 40 preferably is composed of a polymer such as nylon (cg. superpolyamide) or a plasticized polyvinyl chloride which is treated so as to be photosensitive. This polymer layer 40 is originally continuous (not shown), and is then exposed to light through the negative of the image to be reproduced. The portions of the photosensitive polymer layer 40 which are thus exposed to light are hardened to resist an etch solution in which the plate is placed after it has been exposed. Thus, when the polymer is subjected to the etching process, the non-image areas of the polymer are etched away to the adhesive layer 42, thereby forming the raised image areas 14 on the printing plate 16.

It will be understood, however, that the specific construction of the printing plate 16 and the manner in which the printing surfaces 17 are formed comprises no part of the instant invention. If desired, the depressed, nonprinting portions of the plate 40 may be produced mechanically.

As shown in FIG. 3, the adhesive layer 42, being nonphotosensitive, resists the etch solution so that neither it nor the steel inner layer 38 are etched at all. This prevents panelling of the inner layer 38 and thereby insures that the inner layer 38 will hug the base cylinder 1- so as to be concentric with the outer surface thereof. The concentricity of the inner layer 38 and the inherent flexibility of the polymer layer 40 result in curved printing surfaces 17 on the image areas 44 which are also concentric with the cylindrical surface of the cylinder 14. This concentricity results in uniform pressure between the printing plate surfaces 17 and the blanket to produce an even transfer of the ink 19 to the blanket 20 and thus to the sheets 26. Furthermore, the plastic or polymer printing surface 17 carries more ink 19 and thus allows more ink to be transferred to the blanket 20 and the sheets 26 than would a conventional metallic printing surface, thereby producing sharply detailed prints of superior color on the sheets Experimentation has shown that a quality print on metal may be achieved with the instant dry offset apparatus by utilizing a printing plate having a total thickness of .030 inch, and comprising .008 inch of a stainless steel 4 layer 33, .006 inch of an adhesive layer 42 and .016 inch of a photosensitive polymer layer 40. The squeeze between the inking form rollers 10 and the printing plate 16 is maintained within the range of .003 to .030 inch so as to provide firm contact between the form rollers 10 and the printing plate 16, while the surfaces of the form rollers 10 are preferably somewhat soft and unglazed. In this manner, both the image and non-image areas of the printing plate 16 are inked, and it is possible to obtain even ink distribution on the printing plate 16, thereby avoiding a halo condition on the printed work resulting from uneven ink distribution thereon. The squeeze between the printing plate 16 and the blanket 20 and between the blanket 20 and the sheet 26 is preferably maintained in the range of .001 inch to .008 inch. It is noted that these dimensions are not intended to limit the present invention, but are merely presented to illustrate a spe ific example of a working embodiment thereof.

It is thought that the invention and many of its attendadvantages will be understood from the foregoing descri tion, and it will be apparent that various changes may be made in the form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred embodiment thereof.

I claim: A method of printing an image on a metallic surface, comprising the steps of:

applying a printing plate having raised image areas of plastic material and recessed non-image areas to the outer surface of a printing cylinder, said image areas having the outer surface thereof disposed in concentric relation to the outer surface of the printing cylinder, bringing inking form rollers into firm contact with the printing plate to ink both the raised image areas and the recessed non-image areas thereof and to obtain a sufiicient and even distribution of ink on the image areas, lightly contacting only the inked, raised image areas of the printing plate with a blanket cylinder to trans fer the ink on the image areas thereto, and bringing the blanket cylinder into engagement with the metallic surface to transfer the ink from the image areas to the surface, thereby reproducing the image thereon.

References Cited by the Examiner UNITED STATES PATENTS 6/31 Christian 101217 8/56 Plambeck 101401.1 X 10/62 Burg et a1. 

